We give an explicit construct of a harmonic weak Maass form $F_{Theta}$ thatis a "lift" of $Theta^3$, where $Theta$ is the classical Jacobi thetafunction. Just as the Fourier coefficients of $Theta^3$ are related to classnumbers of imaginary quadratic fields, the Fourier coefficients of the"holomorphic part" of $F_{Theta}$ are associated to class numbers of realquadratic fields.
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